It is customary, in the circulation of the molding sand of a foundry using clay-bonded, green-molded casting sand, for the largest part of the used sand obtained at the place of mold unpacking to be fed through a processing installation for reuse in the green-sand molding. This used sand is a mixture of primarily clay-bound molding sand and smaller quantities of chemically bound core sand which had been introduced into circulation for the first time as new sand by way of the core-making installation. The used sand regularly contains still active bonding clay (bentonite) as well as carbonaceous residues, especially coked, porous coal dust. In addition, the grains of sand are increasingly changed in a system involving repeated circulation because a part of the bonding clay is burned dead (calcined) as a result of the heat action of the casting metal, and adheres as a ceramic, porous surface layer on the grains of quartz, a phenomenon referred to as oolitization.
The above-mentioned processing takes these circumstances into account in the return of the used sand. The active bentonite contained in the used sand is made bondable again with the addition of new bonding clay and water. Oolitization and coal dust have favorable effects on the characteristics of the molding substance up to a certain degree.
However, not all of the used sand can be reused in this manner. New quartz sand is introduced continuously into the system predominantly by its initial use in the core shop. Used sand must be separated out to a corresponding degree (apart from uncontrollable losses) because the requirement for clay-bonded molding sand remains, on the average, rather constant. The hauling away and the disposition of this quantity of used sand (waste sand) causes considerable expense and is also a burden on the environment.
It would therefore be desirable to be able to use such used sand instead of new sand. However, this has not been possible because of the significant deviation in the nature of the used sand from the new sand as described above: active, mostly basic bentonite is incompatible with practically all of the chemically setting binding systems used in core production. In addition, and because of the porosity of the oolitic shells of the grains and of the carbonaceous granules as well as because of the high fine particulate matter (also known as fines or AFS clay) content, the consumption of liquid chemical binders would be much too high. It is therefore quite clear that regeneration of the used sand for reuse with chemical binders in the core shop is much more difficult than the previously mentioned customary processing with binder clay and water. Thus, in order that the used sand can be channeled into reuse replacing new sand, it will be necessary to regenerate it in a manner which imparts to it largely the properties of new quartz sand. A washing process, for example, which merely removes the "fines" will, as a rule, not achieve the goal.
A proposed regeneration process is described in German Offenlegungsschriften 22 52 217 and 22 52 259 wherein used sand is first reduced to grain size, and it is then calcined at a temperature of between about 550.degree. and 1,300.degree. C., and, finally, it is subjected to a cleaning of the grains by mechanical and/or pneumatic rubbing together of the grains. This, however, requires a considerable expenditure in machine installations through which the material must pass in sequence. Furthermore, the energy requirement is considerable, particularly for the annealing. It is moreover questionable whether even after the preceding annealing treatment, the casings of clay firmly burnt onto the grains will be sufficiently removed by rubbing.